Permit in-use snapshot exception data to be 'handed over' from onesnapshot instance to another. This is a pre-requisite for patchesthat allow the changes made in a snapshot device to be merged back intoits origin device and also allows device resizing.

The basic call sequence is:

dmsetup load new_snapshot (referencing the existing in-use cow device) - the ctr code detects that the cow is already in use and allows the two snapshot target instances to be linked together dmsetup suspend original_snapshot dmsetup resume new_snapshot - the new_snapshot becomes live, and if anything now tries to access the original one it will receive -EIO dmsetup remove original_snapshot

(There can only be two snapshot targets referencing the same cow devicesimultaneously.)